Glass topped cooking hobs incorporating electric hotplates or gas burners beneath the glass top are known and are usually provided with an over-temperature sensor which effectively senses the temperature of the glass and turns off the hotplate or gas burner when the maximum temperature of the glass has been reached. The most common method of monitoring the glass temperature is to use a rod expansion type thermostat element which extends across the hotplate or gas burner and is preset to an operating temperature and which, when actuated, causes "snap action" electrical contacts to be operated to cause the hotplate or gas burner to be turned off. A cooking hob of this form based on gas burners is disclosed in GB-A-2230595. The use of such thermostat elements suffers from the problem that it is difficult, because of the physical size of the thermostat element, to position the heat sensitive rod part in close contact with the glass, and due to the very poor heat transfer properties of the glass, the temperature actually measured by the thermostat is considerably different to that of the top of the glass, particularly when a cooking utensil is placed on the glass. To ensure that no hazardous condition can occur, such as an aluminium pan reaching its melting point if it is allowed to "boil dry", it is necessary to set the glass overheat thermostat at a considerably lower temperature to achieve the necessary margin of safety. The use of a "snap action" thermostat is necessary to avoid poor contact performance but the inherently wide operating differential with such contacts allows the glass to cool well below its optimum temperature before heat is re-applied.
An alternative method of overheat control is disclosed in GB 1535931. This makes use of a thermocouple element positioned on or very near to the glass for monitoring the glass temperature and for turning off the gas burner should an over-temperature condition be reached. Whilst the use of a thermocouple as proposed in GB 1535931 overcomes some of the problems experienced with rod-expansion type thermostat elements, it suffers from the problem that it only monitors the glass temperature at one particular point and, as has been mentioned, since the glass has very poor heat transfer properties, the overall temperature of the glass over the area of the hotplate or gas burner is not monitored.